The immediate cause of earthquake is believed to be the result of elastic rebound. The explanation has been confirmed over the years. If the rock formations are low in modulus of elasticity, then the maximum deformation is realized before rebound rather than after. During initial energy accumulation stage, there is no change in soil gas radon level. However, radon level starts to increase when stress exceeded one half of the rock strength. Small spike like anomalies start to appear as a result of micro puncture of rock formation and the formation of micro seep when groundwater level is increasing at the same time. The volume of rock starts to increase responding to the development of microfracturing and the flow of groundwater into these fractures. As the microfractures are interconnected, added amount of groundwater will flow into these spaces and result in lowering of groundwater level. The drastic lowering of groundwater level in conjunction with the maximum deformation before rebound produces spike like radon anomaly of short duration. Further interconnecting of the fractures will result in sliding along fault and the rebound. For a stack of mudstone dominant sedimentary rocks, the rebound is small compare with the maximum deformation. After this fast radon release, radon flux starts to decrease while groundwater level is gradually rising responding to the continuing increase of stress after all the fracture spaces are filled. Sometime at this point, the slide along fault occurs and it is the onset of earthquake. The radon release pattern before the onset of an earthquake implies that there must be a degas phase because radon with a molecular weight of 222 is the heaviest natural occurring gas.
Radon anomalies, either in groundwater or as soil gas, have been used as earthquake precursors. However, in actual prediction of earthquakes the precursor is not always effective. One problem is that the signal to background ratio must be improved by placing the detector within a fracture zone of an active fault with upwelling gases. Secondly, the environmental factors affecting radon variation must be reduced, by housing of the detector. Thirdly, the data recording must be continuous and retrievable at a remote site. Thus, there is a need in the art for an improved radon detector that can be used in conjunction with the prediction of earthquakes. There is also a need in the art for a remote radon sensing device that can be used in conjunction with earthquake prediction.
The following patents and publications are part of the background of the invention and are incorporated herein by reference:    (1) Chyi, L. L., Chou, C. Y., Yang, F. T., and Chen, C. H., 2001, Continuous radon measurements in faults and earthquake precursor pattern recognition, Western Pacific Sciences, v. 1, no. 2, 227-245;    (2) Chyi, L. L., Chou, C. Y., Yang, F. T., and Chen, C. H., 2002, Automated radon monitoring of seismicity in a fault zone, Geofisica International, v. 41, no. 4, 507-511;    (3) U.S. Pat. Nos. 5,408,862 and 5,625,138 to Elkins; and    (4) U.S. Pat. No. 5,438,324 to Chyi.